CORROSION PROTECTION WITH AL / ZN-BASED COATINGS

Description

[0001] The present invention relates generally to the production of products that have a coating of an alloy containing aluminium and zinc as the main components of the alloy (hereinafter referred to as "Al/Zn-based alloy coated products").

[0002] The term "Al/Zn-based alloy coated products" is understood herein to include products, by way of example, in the form of strip, tubes, and structural sections, that have a coating of an Al/Zn-based alloy on at least a part of the surface of the products.

[0003] The present invention relates Al/Zn-based alloy coated steel strip according to claim 1. It is noted that unless otherwise specifically mentioned, all references to percentages of elements in the specification are references to percentages by weight.

Background Art

[0004] Thin (i.e. 2-100 µm thick) Al/Zn-based alloy coatings are often formed on the surfaces of steel strip to provide protection against corrosion.

[0005] The Al/Zn-based alloy coatings are generally, but not exclusively, coatings of alloys of elements Al and Zn and one or more of Mg, Si, Fe, Mn, Ni, Sn and other elements such as V, Sr, Ca, Sb in small amounts.

[0006] The Al/Zn-based alloy coatings are generally, but not exclusively, formed on steel strip by hot dip coating strip by passing strip through a bath of molten alloy. The steel strip is typically, but not necessarily exclusively, heated prior to dipping to promote bonding of the alloy to the strip. The alloy subsequently solidifies on the strip and forms a solidified alloy coating as the strip emerges from the molten bath.

[0007] The Al/Zn-based alloy coatings typically have a microstructure consisting predominantly of an Al-rich alpha phase in the form of dendrites and a Zn-rich eutectic phase mixture in the region between the dendrites. When the solidification rate of the molten coatings is suitably controlled (for example, as described in US patent 3,782,909, the Al-rich alpha phase solidifies as dendrites that are sufficiently fine that they define a continuous network of channels in the interdendritic region, and the Zn-rich eutectic phase mixture solidifies in this region.

[0008] WO 2008/141398 A1 discloses a method of solidifying a molten coating of Al-Zn-Si-Mg alloy on the steel strip with a high rate of cooling only after a strong solid supporting network of alpha phase dendrites has been established in the coating and before a Mg₂Si phase has started to form in the coating.

[0009] The performance of these coatings relies on a combination of (a) sacrificial protection of the steel base, initially by the Zn-rich interdendritic eutectic phase mixture and (b) barrier protection by the supporting Al-rich alpha phase dendrites. The Zn-rich interdendritic phase mixture corrodes preferentially to provide sacrificial protection of the steel substrate and, in certain environments, the Al-rich alpha phase can also continue to provide a suitable level of sacrificial protection to the steel substrate, as well as barrier protection, once the Zn-rich interdendritic phase mixture has been exhausted.

[0010] There are, however, many circumstances where the level of barrier protection and sacrificial protection afforded by the Al-rich alpha phase dendrites is insufficient and performance of the coated steel strip may suffer. Three such areas are as follows.

1. In "acid rain" or "polluted" environments containing high concentrations of nitrogen oxides and sulfur oxides.

2. Under paint films in marine environments.

3. At cut edges or other areas where the metallic coating has been damaged to expose the steel substrate in marine environments.

[0011] By way of example, the applicant has found that when Al/Zn-based alloy coatings on steel strip are particularly thin (i.e. coatings having a total coating mass of less than 200, typically less than 150, g per m² of coating, which equates to less than 100, typically less than 75, g per m² of coating on each surface of a steel strip when there are equal coating thicknesses on both surfaces), the microstructure trends to a more columnar or bamboo structure extending from the steel strip to the coating surface when the coating is formed with standard cooling rates, typically from 11°C/s to 100°C/s. This microstructure comprises (a) Al-rich alpha phase dendrites and (b) a Zn-rich eutectic phase mixture forming as a series of separate columnar channels that extend directly from the steel strip to the coating surface.

[0012] WO2008/141398, which is considered to represent the closest prior art, discloses a metal-coated steel strip having a coating of Al-Zn-Si-My alloy solidified on the strip with a high rate of cooling after a sufficiently strong, solid supporting network of alpha phase dendrites has been established in the coating and before a Mg₂Si phase has started to form in the coating, the alloy comprising 40 to 60% by weight aluminium, 40 to 60% by weight Zinc, 0.3 to 3% by weight silicon and 0.3 to 10% by weight Magnesium.

[0013] Other prior art solutions are disclosed in US 4401727 and JP2001-355055.

[0014] The applicant has also found that when steel strip having such thin Al/Zn-based alloy coatings with a columnar microstructure is exposed to low pH environments, commonly described as "acid-rain" environments, or exposed to environments that have high concentrations of sulfur dioxide and nitrogen oxides, commonly described as "polluted" environments, the Zn-rich interdendritic eutectic phase mixture is quickly attacked and the columnar channels of this phase mixture that extend directly from the steel strip to the coating surface act as direct corrosion paths to the steel strip. Where there are such direct corrosion paths from the coating surface to the steel strip, the steel strip is likely to corrode and the corrosion products (oxides of iron) can travel freely to the coating surface and develop an appearance known as "red rust staining". Red rust staining degrades the aesthetic appearance of a coated steel product and can decrease performance of the products. For example, red rust staining can reduce the thermal efficiency of coated steel products that are used as roofing materials.

[0015] The applicant has also found that where the thin Al/Zn-based coating is damaged to reveal the steel strip by scratching, cracking or other means, and exposed to "acid-rain" environments, or "polluted" environments, red rust staining can occur even in the absence of a columnar or bamboo structure.

[0016] It is also known that in an "acid rain" environment or a "polluted" environment the Al-rich alpha phase is unable to sacrificially protect the steel strip.

[0017] An "acid rain" environment is understood herein to be an environment where the rain and/or condensation forming on a coated steel strip has a pH of less than 5.6. By way of example, a "polluted environment" can be typically, but by no means exclusively, defined as a P2 or P3 category in ISO9223.

[0018] Also by way of example, in marine environments, where Al-rich alpha phase dendrites are normally considered to provide good sacrificial protection to a steel substrate, this ability is diminished by changes in the micro-environment beneath paint films applied over the metallic coated steel strip.

[0019] The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

Summary of Invention

[0020] The applicant has found that red rust staining of Al/Zn-based alloy coated steel strip in "acid rain" or "polluted" environments can be prevented or minimised by forming the coating as an Al-Zn-Si-Mg alloy coating and ensuring that the OT:SDAS ratio of the coating is greater than a value of 0.5:1, where OT is the overlay thickness on a surface of the strip and SDAS is the measure of the secondary dendrite arm spacing for the Al-rich alpha phase dendrites in the coating.

[0021] The term "overlay thickness" is understood herein to mean the total thickness of the coating on the strip minus the thickness of the intermetallic alloy layer of the coating, where the intermetallic alloy layer is an Al-Fe-Si-Zn quaternary intermetallic phase layer immediately adjacent to the steel substrate that forms by the reaction between the molten coating and the steel substrate when the coating is applied to the strip.

[0022] According to the present invention there is provided a metal strip according to claim 1.

[0023] The term "Zn-rich eutectic phase mixture" is understood herein to mean a mixture of products of eutectic reactions, with the mixture containing Zn-rich β phase and Mg:Zn compound phases, for example, MgZn₂.

[0024] Preferably, the coating has an OT:SDAS ratio greater than 0.5:1, where OT is the overlay thickness and SDAS is the secondary dendrite arm spacing for the Al-rich alpha phase dendrites of the coating.

[0025] It is noted that, where the coating is on both surfaces of the strip, the overlay thickness on each surface may be different or the same, depending on the requirements for the coated strip. In any event, the disclosure requires that the OT:SDAS ratio be greater than 0.5:1 for the coating on each of the two surfaces.

[0026] The OT:SDAS ratio may be greater than 1:1.

[0027] The OT:SDAS ratio may be greater than 2:1.

[0028] The coating may be a thin coating.

[0029] In this context, a "thin" coating on a metal, such as a steel, strip is understood herein to mean a coating having a total coating mass of less than 200 g per m² coating on both surfaces of the strip, which equates to less than 100 g per m² coating on one surface of the steel strip, which may not always be the case.

[0030] The overlay thickness of the coating may be greater than 3 µm.

[0031] The overlay thickness of the coating may be less than 20 µm.

[0032] The overlay thickness of the coating may be less than 30 µm.

[0033] The overlay thickness of the coating may be 5-20 µm.

[0034] The Al-Zn-Si-Mg alloy may contain 45-60% Al.

[0035] The Al-Zn-Si-Mg alloy may contain 39-48% Zn.

[0036] The Al-Zn-Si-Mg alloy may contain between 1% and 3% Mg.

[0037] The Al-Zn-Si-Mg alloy may contain 1.2-2.8% Mg.

[0038] The Al-Zn-Si-Mg alloy may contain 1.5-2.5% Mg.

[0039] The Al-Zn-Si-Mg alloy may contain 1.7-2.3% Mg.

[0040] The metal strip i is a steel strip.

[0041] In addition or in the event that the above-described OT:SDAS ratio cannot be maintained and the coatings have OT:SDAS ratios of less than 0.5:1, the applicant has also found that red rust staining in "acid rain" or "polluted" environments and also corrosion at cut edges in marine environments can be prevented or minimised in thin Al-Zn-Si-Mg alloy coatings on steel strip by selection of the composition (principally Mg and Si) of the coating alloy and control of the microstructure of the coating.

[0042] The above-described composition selection and microstructure control is particularly useful for thin coatings and/or coatings with an OT:SDAS ratio less than 0.5:1, but is not restricted to these coatings and also applies to thick coatings and/or coatings with an OT:SDAS ratio greater than 0.5:1.

[0043] The applicant has also found that corrosion at cut edges of coated steel strip in marine environments and red rust staining in "acid rain" or "polluted" environments can be eliminated or minimised in susceptible Al/Zn-based coatings by:

1. Blocking corrosion along the Zn-rich interdendritic channels to the steel strip, and/or

2. Rendering the Al-rich alpha phase active in these environments so that it can sacrificially protect the steel strip.

[0044] In general terms, in both cases, according to the present disclosure there is provided a metal strip with a coating of an Al-Zn-Si-Mg alloy on one or both surfaces of the strip that is suitable, by way of example, for "acid rain" or "polluted" environments, with the coating comprising a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, and with particles of Mg₂Si phase in the interdendritic channels.

[0045] The term "particles" is understood herein in the context of Mg₂Si phase to be an indication of the physical form of the precipitates of this phase in the microstructure. It is understood herein that the "particles" form via precipitation from solution during solidification of a coating and are not specific particular additions to the composition.

[0046] The applicant has also found that the improved sacrificial protection that is possible with the present invention applies across a range of microstructures, from coarse dendrite structures with OT:SDAS ratios of 0.5:1 to fine dendrite structures with OT:SDAS ratios of 6:1.

[0047] The applicant has also found that Al-Zn-Si-Mg alloy coated strip manufactured in accordance with the present invention, and subsequently painted, shows the development of a more narrow, uniform corrosion front as a result of Al-rich alpha phase activation and a reduced level of edge undercutting in marine environments.

[0048] Samples manufactured in accordance with the present invention showed a reduced rate of "edge creep" or "undercutting" from cut-edges, compared to conventional Al/Zn coatings, in experimental work carried out by the applicant.

[0049] The improved performance has been shown to apply to a range of coating structures and for a range of paint films.

[0050] The present invention is described further with reference to the accompany drawings, of which:

Figure 1 is a graph of edge undercutting and Mg concentration in examples of Al-Zn-Si-Mg alloy coatings on test samples in marine environments;

Figures 2 to 4 are photographs of test panels and images of corrosion fronts that demonstrate the improved performance of examples of Al-Zn-Si-Mg alloy coatings in marine environments;

Figure 5 are photographs of laboratory accelerated test panels showing improved surface weathering and improved sacrificial protection for metallic coated steel strip

Figures 6 to 11 are photographs of test panels that demonstrate the improved performance of examples of Al-Zn-Si-Mg alloy coatings on steel strip "acid rain" or "polluted" environments;

Figure 12 is a planar view of a scanning electron microscope image of an Al-Zn-Si-Mg alloy coating which illustrates the morphology of Mg₂Si phase particles in the microstructure shown in the image; and

Figure 13 is networked 3-dimensional image of the morphology of Mg₂Si phase particles in the Al-Zn-Si-Mg alloy coating of Figure 12.

[0051] The improved corrosion performance of examples of Al-Zn-Si-Mg alloy coated steel strip has been demonstrated by the applicant on test samples exposed in a range of actual "acid rain", "polluted" and marine environment sites.

[0052] The test samples include test panels developed by the applicant to provide information on corrosion of coatings.

[0053] Figures 1 to 5 and Tables 1 and 2 demonstrate the improved performance of examples of Al-Zn-Si-Mg alloy coatings on steel strip produced in marine environments.

[0054] Performance in marine environments was assessed by outdoor exposure testing at sites with ISO ratings from C2 to C5 as per AS/NZS 1580.457.1.1996 Appendix B and by laboratory Cyclic Corrosion Testing (CCT).

[0055] Table 1 presents data that shows the improved performance in the level of painted edge undercutting of examples of Al-Zn-Si-Mg coated steel test panels for a range of metallic coating mass (unit: mm) for washed exposure in a severe marine environment. The table also includes comparative data for conventional Al/Zn-based alloy coated test panels.

Coating Mass	Edge Undercutting - Conventional Al/Zn Coating	Edge Undercutting - Invention Al/Zn Coating
150g/m2	12	5
100g/m2	20	8
75g/m2	21	9
50g/m2	66	10

[0056] It is evident from Table 1 that there was significantly less edge undercutting with the Al-Zn-Si-Mg coated steel test than with the conventional Al/Zn-based alloy coated test panels.

[0057] Table 2 presents further data that shows the improved performance in the level of undercutting of examples of painted Al-Zn-Si-Mg coated steel test panels for a range of paint types (unit: mm) for washed exposure in a severe marine environment. The table also includes comparative data for conventional Al/Zn-based alloy coated test panels.

Paint Type	Coating Mass	Edge Undercutting - Conventional Al/Zn Coating	Edge Undercutting - Invention Al/Zn Coating
Polyester	150g/m2	9	3.5
Polyester	100g/m2	15	5
Water Based	150g/m2	8	3.2
Water Based	100g/m2	22	4.5
"Cr-Free"	150g/m2	22	6

[0058] It is evident from Table 2 that there was significantly less edge undercutting with the painted Al-Zn-Si-Mg coated steel test panels that with the painted conventional Al/Zn-based alloy coated test panels.

[0059] The photographs of the test panels and the images of the corrosion fronts in Figures 2 to 4 further illustrate the improved performance of examples of Al-Zn-Si-Mg coatings in marine environments. Figure 2 shows improved corrosion performance for fluorocarbon painted, Al-Zn-Si-Mg coatings for unwashed exposure in a severe marine environment. Figure 3 is an example of an extensive corrosion front for a conventional Al/Zn coating under paint in a marine environment. Figure 4 is an example of a narrower and more uniform corrosion front for Al-Zn-Si-Mg coatings under paint in a marine environment

[0060] The photographs of the test panels in Figure 5 demonstrate the improved corrosion performance of examples of Al-Zn-Si-Mg in accelerated test conditions. In particular, Figure 5 shows improved surface weathering and improved sacrificial protection of Al-Zn-Si-Mg coatings in accordance with the present disclosure compared to conventional Al/Zn coatings with coarse or fine structure in a salt fog Cyclic Corrosion and Test.

[0061] Figures 6 to 11 demonstrate the improved performance of Al-Zn-Si-Mg coated steel test panels in "acid rain" or "polluted" environments when produced. The photographs show red rust staining on conventional Al/Zn-based alloy coated steel test panels and no red rust staining on the Al-Zn-Si-Mg coated steel test panels manufactured in accordance with the present invention. Comparison of Figure 9 with Figure 7 shows that the benefit is retained over time. In particular, Figure 6 shows red rust staining on a conventional Al/Zn-based coated steel strip (total coating mass of 100g/m² of coating) exposed in a severe "acid rain" environment for 6 months. Figure 7 shows that there was no red rust staining on an Al-Zn-Si-Mg coating (total coating mass of 100g/m² of coating), exposed in a severe "acid rain" environment for 6 months. Figure 8 shows red rust staining on a conventional Al/Zn-based coated steel strip (total coating mass of 100g/m² of coating), exposed in a severe "acid rain" environment for 18 months. Figure 9 shows that there was no red rust staining on an Al-Zn-Si-Mg coating (total coating mass of 100g/m² of coating), exposed in a severe "acid rain" environment for 18 months. Figure 10 shows that there was red rust staining on a conventional Al/Zn-based coated steel strip with columnar structure (total coating mass of 50g/m² of coating), exposed in a severe "acid rain" environment for 4 months. Figure 11 shows that there was no red rust staining on an Al-Zn-Si-Mg coating with columnar structure (total coating mass of 50g/m² of coating), exposed in a severe "acid rain" environment for 4 months.

[0062] Finally, the applicant found in microstructural analysis of examples of Al-Zn-Si-Mg coatings that the microstructure includes Mg₂Si phase particles of a particular morphology in the interdendritic channels of Zn-rich eutectic phase mixture that are between dendrites of Al-rich alpha phase and this morphology is important in improving the corrosion resistance of the coatings, as discussed above. The applicant found that the size and distribution of the Mg₂Si phase particles are also important factors contributing to the improved corrosion performance of the Al-Zn-Si-Mg coatings in accordance with the present invention. The applicant also found that desirable morphology, size and distribution of Mg₂Si phase particles were possible by selection of coating compositions and control of cooling rates during coating solidification.

[0063] Figures 12 and 13 illustrate one example of the morphology of Mg₂Si phase particles discussed above.

[0064] In the planar image of Figure 12, the darker regions are Al-rich alpha phase dendrites, the bright regions are interdendritic channels with Zn-rich eutectic phase mixture, and the "chinese-script" Mg₂Si phase particles that partially fill the channels.

[0065] In the 3-dimensional image of Figure 13, the Mg₂Si "petals" are shown by the red colour and the other phases include: Si (green), MgZn₂ (blue) and Al-rich alpha phase (dark matrix).

Claims

1. A steel strip with a coating of an Al-Zn-Si-Mg alloy on one or both surfaces of the strip, with the alloy consisting of 40-65% Al, 35-50% Zn, 0.5-2% Si, between 0.5% and 3% Mg, and optionally other elements in small amounts, less than 0.5% for each other element, with all of the percentages being weight percentages, with the coating comprising a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, and with particles of Mg₂Si phase in the interdendritic channels having an appropriate size and morphology that block corrosion along the interdendritic channels, with the volume fraction of interdendritic Mg₂Si phase compared to other Si-containing phases being greater than 50%.

2. A steel strip defined in claim 1 wherein greater than 70% of the total volume fraction of Mg₂Si phase in the coating is in the lower two thirds of the overlay thickness of the coating.

3. A steel strip defined in claim 1 or claim 2 wherein greater than 60% of the interdendritic channels is "blocked" by Mg₂Si phase particles.

4. A steel strip defined in any one of the preceding claims, wherein the coating has an OT:SDAS ratio greater than 0.5:1, where OT is the overlay thickness and SDAS is the secondary dendrite arm spacing for the Al-rich alpha phase dendrites of the coating.

5. A steel strip defined in claim 4 wherein the overlay thickness of the coating is greater than 3 µm.

6. A steel metal strip defined in claim 4 or claim 5 wherein the overlay thickness of the coating is less than 30 µm.

7. A steel strip defined in claim 4 wherein the SDAS of the Al-rich alpha phase dendrites in the coating is greater than 3µm but smaller than 20µm.

Ansprüche

1. Stahlband mit einer Beschichtung aus einer Al-Zn-Si-Mg-Legierung auf einer oder beiden Oberflächen des Bandes, wobei die Legierung aus 40-65 % Al, 35-50 % Zn, 0,5-2 % Si, zwischen 0,5 % und 3 % Mg und optional anderen Elementen in geringen Mengen, geringer als 0,5 % für jedes andere Element, besteht, wobei alle Prozentangaben Gewichtsprozente darstellen, wobei die Beschichtung eine Mikrostruktur umfasst, die Dendriten aus Al-reicher alpha-Phase und interdendritische Kanäle aus Zn-reichem eutektischem Phasengemisch umfasst, die sich von dem Metallband aus erstrecken, und wobei die Partikel der Mg₂Si-Phase in den interdendritischen Kanälen eine geeignete Größe und Morphologie aufweisen, um Korrosion entlang der interdendritischen Kanäle zu blockieren, wobei der Volumenanteil der interdendritischen Mg₂Si-Phase im Vergleich zu anderen Si-haltigen Phasen größer als 50 % ist.

2. Stahlband nach Anspruch 1, wobei mehr als 70 % des Gesamtvolumenanteils der Mg₂Si-Phase in der Beschichtung sich in den unteren beiden Dritteln der Schichtdicke der Beschichtung befinden.

3. Stahlband nach Anspruch 1 oder Anspruch 2, wobei mehr als 60 % der interdendritischen Kanäle durch Partikel der Mg₂Si-Phase "blockiert" sind.

4. Stahlband nach einem der vorhergehenden Ansprüche, wobei die Beschichtung ein OT:SDAS-Verhältnis von mehr als 0,5 : 1 aufweist, wobei OT die Schichtdicke und SDAS der sekundäre Dendritenarmabstand für die Dendriten der Al-reichen alpha-Phase der Beschichtung ist.

5. Stahlband nach Anspruch 4, wobei die Schichtdicke der Beschichtung größer als 3 µm ist.

6. Stahlmetallband nach Anspruch 4 oder Anspruch 5, wobei die Schichtdicke der Beschichtung weniger als 30 µm beträgt.

7. Stahlband nach Anspruch 4, wobei der SDAS der Dendriten der Al-reichen alpha-Phase in der Beschichtung größer als 3 µm, jedoch kleiner als 20 µm ist.

Revendications

1. Bande d'acier avec un revêtement d'alliage Al-Zn-Si-Mg sur une ou les deux surfaces de la bande, l'alliage étant composé de 40 à 65 % d'Al, 35 à 50 % de Zn, 0,5 à 2 % de Si, entre 0,5 % et 3 % de Mg, et facultativement d'autres éléments en faibles quantités, moins de 0,5 % pour chacun des autres éléments, tous les pourcentages étant des pourcentages en poids, le revêtement comprenant une microstructure qui comprend des dendrites de phase alpha riche en Al et des canaux inter-dendritiques de mélange de phase eutectique riche en Zn qui s'étendent depuis la bande métallique, et avec des particules de phase Mg₂Si dans les canaux inter-dendritiques ayant une taille et une morphologie appropriées qui bloquent la corrosion le long des canaux inter-dendritiques, la fraction volumique de phase inter-dentritique Mg₂Si étant supérieure à 50 % comparée à d'autres phases contenant du Si.

2. Bande d'acier selon la revendication 1, dans laquelle plus de 70 % de la fraction volumique totale de phase Mg₂Si dans le revêtement se trouve dans les deux tiers inférieurs de l'épaisseur de couverture du revêtement.

3. Bande d'acier selon la revendication 1 ou la revendication 2, dans laquelle plus de 60 % des canaux inter-dendritiques sont « bloqués » par des particules de phase Mg₂Si.

4. Bande d'acier selon l'une quelconque des revendications précédentes, dans laquelle le revêtement a un rapport OT:SDAS supérieur à 0,5:1, où OT est l'épaisseur de couverture et SDAS est l'espacement secondaire de bras de dendrite pour les dendrites de phase alpha riche en Al du revêtement.

5. Bande d'acier selon la revendication 4, dans laquelle l'épaisseur de couverture du revêtement est supérieure à 3 µm.

6. Bande métallique d'acier selon la revendication 4 ou la revendication 5, dans laquelle l'épaisseur de couverture du revêtement est inférieure à 30 µm.

7. Bande d'acier selon la revendication 4, dans laquelle le SDAS des dendrites de phase alpha riche en Al dans le revêtement est supérieur à 3 µm mais inférieur à 20 µm.

Drawing